In cellular radio-telephone systems, synchronization acquisition is the first operation to be performed by a terminal once it has been switched on. This operation generally comprises two stages:
a frequency synchronization first stage that consists in acquiring the frequency reference of the base station; and PA1 a time synchronization second stage that consists in acquiring the clock of the base station. PA1 the synchronization signal which conveys useful information while telephone calls are being set up or received (cell identity, pilot signal power, and initial time offset of the spreading sequence). PA1 the signals coming from the satellite are affected by considerable Doppler displacement because of the high orbiting speed of the satellite; and PA1 the distance between the mobile station and the satellite in low Earth orbit (LEO) varies by several thousand kilometers in a few minutes, and, as a result, the instantaneous variation in propagation time is considerable. PA1 the first packet, which is referred to as the "FCCH" (frequency correction channel), enables the mobile station to acquire the frequency reference of the base station; this packet contains a pure sine-wave; and PA1 the second packet, which is referred to as the "SCH" (synchronization channel), enables the mobile station to acquire the time reference; this packet contains a binary sequence which has advantageous autocorrelation properties. PA1 a sampler delivering series of samples x(i) of said synchronization signal corresponding to said sequences; PA1 a first shift register comprising N cells, fed by said sampler; PA1 a second shift register comprising N cells, fed by the output of said first shift register; PA1 multiplication means for multiplying in pairs the contents of the same-rank cells of said first shift register and of said second shift register, the multiplication means delivering N values c(0) to c(N-1); and PA1 spectrum analysis means for performing spectrum analysis on said N values, which means deliver both first information representing a time reference, and also second information representing the frequency displacement, e.g. the Doppler displacement. PA1 receiving 2N samples respectively corresponding to the N.sub.s digital elements of said first sequence s(0) to s(N.sub.s -1) and to the N.sub.s digital elements of said second sequence s(N.sub.s -1) to s(0); PA1 multiplying in pairs the same-rank digital elements of each of said sequences, thereby delivering N values c(i)=x(i)*x(N-1-i); and PA1 performing spectrum analysis on said N values, so as to determine both first information representing a time reference, and also second information representing the frequency displacement, e.g. the Doppler displacement. PA1 T.sub.e is the sampling period of said synchronization signal; PA1 .delta. represents said frequency displacement; and PA1 .PHI. represents said time reference; PA1 analysis of the first term of the equation (1), corresponding to a spread spectrum signal, so as to determine said time reference; and PA1 analysis of the second term of the equation (1), corresponding to a sine-wave of frequency 2.delta., so as to determine said information representing said Doppler displacement.
Conventionally, these two operations are independent from each other. They use reference signals that are distinct and independent.
In this way, in a code division multiple access system, each base station broadcasts two signals intended for acquiring and maintaining synchronization, namely: the pilot signal constituted by continuous transmission of a sequence of pseudo-random noise PN serving to spread data, and not modulated by information; this signal, which is tracked continuously by the mobile station, guarantees that time and frequency synchronization is maintained; and
That technique is complex and it is not adapted to satellite broadcast systems. For example, adapting a CDMA system to the Globalstar system poses two major problems:
Likewise, in a time division multiple access system, such as the GSM system, initial synchronization is performed by means of two specific packets transmitted at regular time intervals over the BCCH (broadcast control channel) carrier:
Those two packets, which are broadcast regularly, occupy non-negligible resources on the BCCH carrier.
Furthermore, it is known that, when a transmitter (base station or mobile station) transmits a sequence of symbols over a transmission channel, the transmitted sequence is degraded, so that the sequence of symbols as received by the receiver is not identical to the sequence as transmitted. The main source of such degradation is the interference between symbols that occurs due to the fact that a transmitted symbol may take any one of several paths in the transmission channel (multiple reflections off surrounding objects). When the time interval between two possible paths is greater than the duration of a symbol, two successive symbols might interfere with each other.
At the receiver, in order to correct the interference between symbols, it is necessary to use an equalizer which, in order to operate properly, must know the impulse response of the transmission channel.
In the GSM system, a sequence of specific symbols referred to as the "training sequence" is inserted into the middle of each packet that is transmitted. The training sequence is defined as a function of the characteristics of the transmission channel, and, in particular, of its length L as defined by the time interval between the shortest path and the longest path (the length of the transmission channel may be expressed in number of symbol durations).
In the GSM system, the impulse response h(t) of the channel is calculated as follows: the receiver has a replica of the training sequence used, and it correlates the replica with the corresponding received sequence.
The result of this cross-correlation constitutes a set of coefficients h(i) (where i varies over the range 0 to L) that are intended to be fed to the equalizer. The shortest path corresponds to h(0). Channel estimation is performed after synchronization.
That known technique suffers from the major drawback that a training sequence must be provided in each data packet, to the detriment of the useful data-rate.